Paint used in forming a magnetic recording medium backcoat layer

ABSTRACT

A magnetic recording medium comprising a magnetic layer, a backing layer, and a support sandwiched between the magnetic layer and the backing layer. The backing layer is made form a coating composition comprising a filler, binder, and a silicone compound represented by the following formula (I): ##STR1## wherein groups A, B, and D each independently represent --CH 3  or a group -X-Y-Z, in which X represents a C1-C11 alkylene or --(CH 2 ) 3  NHCO--, Y represents a specific aliphatic group, alicyclic group, or an aromatic group, and Z represents --OH, m is an integer from 4 to about 150, and n is an integer from 1 to 6, 
     the compound of formula (I) containing at least one -X-Y-Z group, with the ratio of the number of groups -X-Y-Z to m being from 1:3 to 1:about 30.

This application is a divisional of U.S. patent application Ser. No.08/346,738, filed Nov. 30, 1994 now U.S. Pat. No. 5,637,405.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a magnetic recording medium, and moreparticularly, to a magnetic recording medium including a backing layerwhich has a very smooth and flat surface and which does not adhere to amagnetic layer when the medium is stored over a long period.

2) Related Art

Recently, as recording density of magnetic recording media is enhanced,the surfaces of their magnetic layers have become more mirror-like,thereby making stable traveling of magnetic tapes difficult. Inpractice, therefore, a backing layer is provided on the opposite surfaceof a support with respect to a magnetic layer to ensure stable travelingperformance of a magnetic recording medium. In order to prevent asurface-form of the backing layer from being transcribed to the magneticlayer, there is acute need for smoothness of the backing layer itself.However, if the magnetic layer and the backing layer both have smoothand flat surfaces, the two layers adhere to each other after themagnetic recording medium is stored in a wound state for a long time.This phenomenon causes damages to the magnetic layer, so as to lead toincrease in dropout and occurrence of jittering of images.

In addition, as portable video cameras have been popular, they have cometo be used under a variety of situations such as an outdoor field, etc.As a result, it is necessary to prevent adhesion when the cameras areexposed to severe environments such as high temperature and highhumidity over long periods.

To prevent adhesion, addition of a silicone oil is known. However, sincethe silicone oil generally has poor compatibility with binders, it isliable to cause defects in coating films such as repelling, pin holes,and the like. Proposed is an attempt for improving compatibility betweensilicone oils and binders by denaturing the silicone oils using fattyacids (JP-A-61-273720). This is effective as long as the compatibilityis concerned. However, since the silicone oil and the binder are notfixed to each other, the silicone oil transfers to the magnetic layerwhen stored in a wound state for a long period. Thus, preventingadhesion does not last long. Furthermore, the silicone oil transferredto the magnetic layer often affects electromagnetic characteristics ofthe magnetic layer. Another proposal is to use a reaction product of areactive polysiloxane and a reactive resin. This approach is notsuccessful in achieving sufficient non-adhesion property, because thepolysiloxane is taken into the resin so as to hinder molecularorientation of the polysiloxane onto the surface of the backing layer.

The inventors of the present invention have conducted extensive studiesto solve the above problems, and they have invented the magneticrecording medium as described below.

An objection of the invention to provide a magnetic recording mediumwhich has a smooth and flat surface.

Another object of the invention is to provide a recording medium whichhardly has pinholes nor occurs repelling.

The other object of the invention is to provide a magnetic recordingmedium having sufficient durability of its backing layer.

A futher object of the invention is to provide a paint for producing abacking layer of the above medium.

SUMMARY OF THE INVENTION

In the first aspect, the present invention is a magnetic recordingmedium comprising a magnetic layer, a backing layer, and a supportsandwiched between the magnetic layer and the backing layer, wherein thebacking layer is made from a coating composition comprising a filler,binder, and a silicone compound represented by the following formula(I): ##STR2## wherein groups A, B, and D each independently represent--CH₃ or a group -X-Y-Z, in which X represents a C1-C11 alkylene or--(CH₂)₃ NHCO--, Y represents an aliphatic group, alicyclic group, or anaromatic group having at least 3 carboxylate ester bonds therein and anumber-average molecular weight Mn of from about 300 to about 3,000, andZ represents --OH, m is an integer from 4 to about 150, and n is aninteger from 1 to 6,

the compound of formula (I) containing at least one -X-Y-Z group, withthe ratio of the number of groups -X-Y-Z to m being from 1:3 to 1:about30.

In the second aspect, the invention is a paint for forming a backinglayer which is provided on/over one surface of a magnetic recordingmedium wherein on/over the other surface thereof a magnetic layer isprovided, comprising a filler, a binder, a dispersant and the siliconecompound of formula (I).

DESCRIPTION OF PREFERRED EMBODIMENTS

The fillers which can be used in the present invention are those whichare generally used in backing layers, and include carbon black,graphite, calcium carbonate, barium sulfate, titanium oxide, alpha-ironoxide, plate-like alpha-iron oxide, alumina, zirconia, silicon carbide,silicon oxide, chromium oxide, magnesium carbonate, zinc oxide, silica,etc. From the viewpoints of conductivity and light shielding, carbonblack is preferable. Specific examples of carbon black includeBLACKPEARLS 1400, 1300, 1100, 800, MONARCH 1400, 1300, 1100, 800, 120,VULCAN-XC 72, BLACKPEARLS L (ex Cabot); #3750, #3950, #2600, #2400,#990, #960, #850, MA 600, MA 7, MA 100, MA 100R (ex Mitsubishi KaseiCorp.); CONDUCTEX-SC, CONDUCTEX 975, RABEN 7000, 5000, 3500, 2000, 1500,1255, 1250, 1170, 1060, 1020, 760, 450, SEVACARB MT (ex CorenbianCarbon); #80, #55, #50, #35, #15, ASAHIMARSAL (ex Asahi Carbon); Shiest9H, 7H, 5H (ex Tokai Carbon Co., Ltd); Ketchen Black EC600JD (ex Akzo);and Thermax MT-N990 (ex Cancarv).

An average first particle size of the carbon black, which is limited tospecific one, is preferably 10-40 nm for surface smoothness andflatness. Adjustment of a surface coarse level, traveling property anddurability are improved by mixing carbon blacks having an average firstparticle size of 10-40 nm and 40-500 nm, respectively, in a ratio of theformer to the latter of from 99.5:0.5 to 30:70. Durability can also beimproved by using both carbon black and an inorganic pigment such astitanium oxide, calcium carbonate, alpha iron oxide, barium sulfate.

The binders which can be used in the present invention are knownthermoplastic resins, thermosetting resins, reactive resins, celluloseresins, and mixtures thereof. The thermoplastic resins are those havinga glass transition temperature of from -100° to 150° C. and anumber-average molecular weight of from 1,000 to 200,000, preferablyfrom 10,000 to 100,000. Examples thereof include polymers and copolymerscontaining, as their monomer units, vinyl chloride, vinyl acetate, vinylalcohol, maleic acid, acrylic acid, acrylic esters, vinylidene chloride,acrylonitrile, styrene, vinyl butyral, or vinyl acetal; polyurethaneresins; polyester resins; and various resins of a rubber type. Examplesof the thermosetting resins include phenol resins, epoxy resins, phenoxyresins, polyurethane setting resins, urea resins, melamine resins, alkydresins, acrylic resins, formaldehyde resins, silicone resins, mixturesof a polyester resin and an isocyanate prepolymer, mixtures of polyesterpolyol and polyisocyanate, and mixtures of polyurethane andpolyisocyanate. Examples of fiber resins include nitrocellulose,ethylcellulose, cellulose acetate butyrate, and cellulose acetatepropionate.

The above resins are used alone or in combination. Preferred arecombinations of a polyurethane resin and a member selected from thegroup consisting of vinyl chloride resins, vinyl chloride/vinyl acetateresins, vinyl chloride/vinyl acetate/vinyl alcohol resins, vinylchloride/vinyl acetate/maleic anhydride resins, vinyl chloride/acrylicester resins, nitrocellulose, and phenoxy resins; and combinationsfurther including polyisocyanate. It is more preferable to use acombination of nitrocellulose, a urethane resin and polyisocyanate; anda combination of a vinyl chloride resin, a urethane resin and apolyisocyanate.

Preferable examples of the vinyl chloride copolymers include thosecontaining a repeating unit of vinyl chloride, a repeating unit havingan epoxy group, and a repeating unit containing a polar group such as--SO₃ M, --OSO₃ M, COOM, --PO(OM)₂ (M is hydrogen or an alkali metal).

The urethane resins which can be used in the invention acre known onessuch as polyester polyurethane, polyether polyurethane, polycarbonatepolyurethane, and polycaprolactone polyurethane. They may optionallyhave at least one polar group selected from the group consisting of--COOM, --SO₃ M, --OSO₃ M, --PO(OM)₂ (M is hydrogen or alkali metal),--OH, NR₂, N+R₃ (R is hydrocarbon), epoxy, and sulfobetaine.

Specific examples of the polyisocyanate include tolylene diisocyanate,4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylenediisocyanate, orthotoluidine isocyanate, isophorone diisocyanate, andtriphenylmethane triisocyanate; reaction products of such an isocyanateas above and a polyalcohol; and polyisocyanates obtained fromcondensation reaction of isocyanates.

The polyisocyantes react with the polar groups in a binder and achievethree-dimensional cross-linking, thereby enhancing the strength anddurability of the coating. In addition, they react with --OH in thesilicone compound of the present invention, and thus they immobilize thesilicone compound onto the surface of the coating.

In the group -X-Y-Z of the silicone compound which can be used in theinvention, Y is an important part of the silicone compound. The group Yis a carboxylate ester group having at least 3 groups represented by thefollowing formula. ##STR3## If the number of carboxylate ester groupscontained in group Y is 1 or 2, sufficient compatibility with variousbinders cannot be obtained. These groups are linked to each other by adivalent hydrocarbon group having 2 to 12, and preferably 4 to 6 carbonatoms. Saturated aliphatic hydrocarbon having 5 carbon atoms isespecially preferred. Group Y is preferably a polycaprolactone formed bypolymerization of caprolactone, as described below. Group Y preferablyhas a number-average molecular weight of 300 to 3,000, and morepreferably 500 to 1,500.

In the group -X-Y-Z of the silicone compound used in the presentinvention, X is preferably a C1-C11 alkylene or --(CH₂)₃ NHCO-- and Z ispreferably --OH. Z may be --COOH, --NCO, or --CH═CH₂ ; and --OH is themost preferred for Z in view of the stability of the resultant compound,compatibility with the binder, and reactivity. The hydroxyl value of thesilicone compound containing --OH is preferably not less than 5(KOHmg/g). If the hydroxyl value is less than 5 (KOHmg/g), reaction withthe binder proceeds insufficiently so as to allow the silicone compoundto transfer into a magnetic layer. As a result, adhesion preventioneffect does not last for a long time.

In the silicone compound used in the present invention, each of groups Aand B is preferably a group -X-Y-Z, and group D is preferably --CH₃.This is a so-called linear siloxane. Another preferable example is onein which each of groups A and B is --CH₃ and group D is a group -X-Y-Z.The ratio of the number of groups -X-Y-Z to m is preferably from 1:3 to1:about 30, and more preferably from 1:4 to 1:16. Ratios greater than1:about 30 considerably result in reduction of the compatibility withthe binder.

In the silicone compound used in the present invention, m is preferablyan integer from 4 to about 150, and n is preferably an integer from 1 to6.

The silicone compound used in the invention can be prepared from areactive polysiloxane containing --OH, --COOH, and --NH₂. The equivalentamount of a starting polysiloxane compound required for the reaction forobtaining an ester group-containing olysiloxane is decided on the basisof determination of a hydroxyl value, acid value, or an amine value. Thereaction between a lactone such as beta-propiolactone,delta-valerolactone, epsilon-caprolactone, or dodecalactone and areactive siloxane containing --OH is carried out by a ring-openingesterification method. The --OH group required for initiatingring-opening polymerization is supplied from the reactive siloxanecontaining --OH. The chain length of the polyester is changed as afunction of mol numbers of the lactone consumed by the hydroxyl group.The lactone polymerization proceeds, for example, as described in thefollowing scheme (II), in a suitable solvent such as a high boilingpoint fraction of gasoline distillate, an alkyl benzene, an ester or aketone, or in its own molten state, at about 100° to about 180° C., inthe presence of paratoluenesulfonic acid or dibutyric tindilaurate.##STR4##

The silicone compound used in the present invention is fixed or orientedonto the surface of the coating, when the silicone coating is dried.Therefore, a remarkable advantage can be effected by use of a smallamount of the silicone compound. In the composition of the backinglayer, the amount of the silicone compound of the invention is from 0.05to 10.0% and preferably from 0.1 to 5.0%, based on the non-volatilecomponents of the composition. If the amount of the silicone compound isless than 0.05%, prevention of adhesion is insufficient, whereas if theamount exceeds 10.0%, smoothness and flatness of the coating arereduced.

The coating according to the present invention can be manufactured byconventional methods. For example, a solution is obtained by dissolvinga portion or the entire amount of the binder in an organic solvent suchas methyl ethyl ketone, toluene, methylisobutylketone, butyl acetate,cyclohexanone, tetrahydrofuran. The filler is then added to thesolution. The resultant mixture is stirred using any of various kinds ofdispersing apparatus such as a double roll mill, a triple roll mill, asand mill, an atriter, a kneader, or a dissolver to prepare dispersion.The silicone compound used in the invention can be added either duringor after the dispersing operation. During dispersion, an organiccolorant represented by the following formula (Ca) or (Cb) is preferablyused as a dispersant. ##STR5## wherein Q is an organic colorant residue;X is a direct bond, CONH--Y₁ --, --SO₂ NH--Y₁, or CH₂ NHCOCH₂ NH--Y₁ --in which Y₂ is an alkylene or arylene group which may have asubstituent;

Y is a substituent represented by the following formula: ##STR6##

R₁ and R₂ are each independently substituted or unsubstituted alkyl, orR₁ and R₂ may be linked to form at least one hetero ring;

Y₂ is hydrogen, a halogen atom, NO₂, NH₂ or SO₃ H;

k is an integer of 1-4; m is an integer of 1-6; and n is an integer of1-4. ##STR7## wherein Q is an organic colorant residue; X is a directbond, CONH--Y₂ --, --SO₂ NH--Y₂, or CH₂ NHCOCH₂ NH--Y₂ -- in which Y₂ isan alkylene or arylene group;

Y₁ is --NH or --O--,

Z is hydroxy, alkoxy or ##STR8## (in which Y₃ is NH or O), or --NH--X-Qwhen n is one;

R₁ and R₂ are each independently substituted or unsubstituted alkyl, orR₁ and R₂ may be linked to form at least one hetero ring;

m is an integer of 1-6; and n is an integer of 1-4.

The organic colorant residue includes a pigment or colorant such asphthalocyanine, azo, anthraquinone, quinaquidone, dioxazine,anthrapyrimidine, anthanthron, indanthron, flavanthron, perylene,perynone, thioindigo, isoindolinone, triphenylmethane, andbenzimidazolone types. These compounds are particularly effective forobtaining a smooth and flat surface in case that carbonblack is used asthe filler. The colorant may be used alone, as a dispersant, or incombination with a dispersant such as a coupling agent, or surfactant.The coating composition may optionally contain known lubricants, settingcatalysts and anti-mold agents. The polyisocyanates are preferably addedto and mixed with the coating composition immediately before applyingthe coating composition onto a magnetic recording medium.

In the invention, the amount of the organic colorant is preferably from0.1 to 50 parts, more preferably from 0.5 to 30 parts by weight per 100parts by weight of the filler. The ratio of the filler to the binder isfrom 100:5 to 100:300, more preferably from 100:10 to 100:200 on thebasis of solid weight thereof.

Preparation Examples of the silicone compound used in the presentinvention are described hereinbelow.

PREPARATION EXAMPLE 1

Into a reactor equipped with a stirrer and a reflux condenser, addedwere polysiloxane (0.5 mol) of the following average formula having ahydroxyl value of 99 (KOHmg/g): ##STR9## and dibutyl tinlaurate (100ppm) so as to react them with epsilon-caprolactone (5 mol) in nitrogenatmosphere at 160° C. After the reaction for 8 hours, a siliconecompound A having the following average formula was obtained. ##STR10##

The silicone compound A had a hydroxyl value of 45 (KOHmg/g) and asaponification value of 240 (KOHmg/g).

PREPARATION EXAMPLE 2

In the same manner as described in Preparation Example 1, polysiloxane(0.5 mol) of the following average formula having a hydroxyl value of143 (KOHmg/g): ##STR11## was reacted with epsilon-caprolactone (4 mol)to obtain a silicone compound B having the following average formula.##STR12##

The silicone compound B had a hydroxyl value of 60 (KOHmg/g) and asaponification value of 256 (KOHmg/g).

PREPARATION EXAMPLE 3

In the same manner as described in Preparation Example 1, polysiloxane(0.3 mol) of the following average formula having a hydroxyl value of 55(KOHmg/g): ##STR13## was reacted with epsilon-caprolactone (9 mol) toobtain a silicone compound C having the following average formula.##STR14##

Silicone compound C had a hydroxyl value of 15 (KOHmg/g) and asaponification value of 340 (KOHmg/g).

PREPARATION EXAMPLE 4

In the same manner as described in Preparation Example 1, polysiloxane(0.3 mol) of the following average formula having a hydroxyl value of 50(KOHmg/g): ##STR15## was reacted with epsilon-caprolactone (8 mol) toobtain a silicone compound D having the following average formula.##STR16##

The silicone compound D had a hydroxyl value of 35 (KOHmg/g) and asaponification value of 240 (KOHmg/g).

PREPARATION EXAMPLE 5

In the same manner as described in Preparation Example 1, polysiloxane(0.3 mol) of the following average formula having a hydroxyl value of 60(KOHmg/g): ##STR17## was reacted with epsilon-caprolactone (10 mol) toobtain a silicone compound E having the following average formula.##STR18##

The silicone compound E had a hydroxyl value of 25 (KOHmg/g) and asaponification value of 280 (KOHmg/g).

PREPARATION EXAMPLE 6

In the same manner as described in Preparation Example 1, polysiloxane(0.3 mol) of the following average formula having a hydroxyl value of 15(KOHmg/g): ##STR19## was reacted with epsilon-caprolactone (9 mol) toobtain a silicone compound F having the following average formula.##STR20##

The silicone compound F had a hydroxyl value of 4 (KOHmg/g) and asaponification value of 200 (KOHmg/g).

PREPARATION EXAMPLE 7

In the same manner as described in Preparation Example 1, polysiloxane(0.5 mol) of the following average formula having a hydroxyl value of 70(KOHmg/g): ##STR21## was reacted with epsilon-caprolactone (1 mol) toobtain a silicone compound G having the following average formula.##STR22##

The silicone compound G had a hydroxyl value of 20 (KOHmg/g) and asaponification value of 100 (KOHmg/g).

EXAMPLES

The present invention will next be described by way of examples. In theExamples, the terms "parts" and "%" indicate "parts by weight" and "% byweight", respectively. The amounts of resins refer to those of solidresins.

Example 1

    ______________________________________    Metallic iron magnetic powder (specific surface area: 50                               100    parts    m.sup.2 /g, coercive force: 1500 Oe)    Vinyl chloride resin (MR-110 ex Nippon Zeon)                               10     parts    Urethane resin (Takerakku E-760, ex Takeda Chemical                               5      parts    Industries, Ltd)    Alpha-alumina (particle size: 0.5μ)                               3      parts    Carbon black (Conductex SC, ex Columbia Carbon)                               5      parts    Stearic acid               1      part    Butyl stearate             1      part    Methyl ethyl ketone        120    parts    Toluene                    120    parts    Cyclohexanone              60     parts    ______________________________________

The above components were dispersed with a sand mill to prepare amagnetic coating composition. Three parts of a polyisocyanate compound(Coronate L, Nippon Polyurethane) were added to the composition. Theresulting mixture was applied onto a support made from polyethyleneterephthalate having a thickness of 10 μm. Molecular orientation wasconducted, followed by drying. Subsequently, mirror-processing wasperformed by a calender process to obtain a magnetic coat having athickness of 3 μm.

Separately, a back-coating composition of the following formulation waspretreated and dispersed by using a sand mill. To the resultingdispersion, 20 parts of Coronate L were further added to obtain carbonblack dispersion. This dispersion was applied to the opposite surface ofthe support with respect to the magnetic layer, dried to form a backinglayer having a thickness of 0.7 μm, and subjected to a setting processat 60° C. for 20 hours. It was cut. Thus, a magnetic tape having a widthof a half inch was obtained.

    ______________________________________    Carbon black (Blackpearls 800, ex Cabot)                               100    parts    Silicone compound A (prepared in Preparation Example 1)                               3      parts    Nitrocellulose (Celluline FM-200S, ex Dicel Chemical                               60     parts    Industries, Ltd)    Urethane resin (Takeluck E-760, ex Takeda Pharmaceutical                               40     parts    Chemical Industies, Ltd    Organic colorant CuPc-SO.sub.2 NH(CH.sub.2).sub.3 N(CH.sub.2 --CH.sub.3).s    ub.2                       5      parts    (CuPc: phthalocyanine)    Methyl ethyl ketone        400    parts    Toluene                    400    parts    Cyclohexanone              200    parts    ______________________________________

Example 2

A magnetic tape was produced in the same manner as in Example 1, exceptthat silicone compound A in the back-coating composition of Example 1was replaced by silicone compound B.

Example 3

A magnetic tape was produced in the same manner as in Example 1, exceptthat silicone compound A in the back-coating composition of Example 1was replaced by silicone compound C prepared in Preparation Example 3.

Example 4

A magnetic tape was produced in the same manner as in Example 1 exceptthat silicone compound A in the back-coating composition of Example 1was replaced by silicone compound D prepared in Preparation Example 4.

Example 5

A magnetic tape was produced in the same manner as in Example 1 exceptthat silicone compound A in the back-coating composition of Example 1was replaced by silicone compound E prepared in Preparation Example 5.

Example 6

A magnetic tape was produced in the same manner as in Example 1 exceptthat the nitrocellulose in the back-coating composition of Example 1 wasreplaced by a vinyl chloride resin (MR-110, Nippon Zeon).

Example 7

A magnetic tape was produced in the same manner as in Example 1 exceptthat the organic colorant in the back-coating composition of Example 1was eliminated.

Comparative Example 1

A magnetic tape was produced in the same manner as in Example 1 exceptthat silicone compound A in the back-coating composition of Example 1was eliminated.

Comparative Example 2

A magnetic tape was produced in the same manner as in Example 1 expectthat silicone compound A in the back-coating composition of Example 1was replaced by a reactive polysiloxane having the following averageformula. ##STR23##

Comparative Example 3

A magnetic tape was produced in the same manner as in Example I exceptthat silicone compound A in the back-coating composition of Example 1was replaced by a reactive polysiloxane having the following averageformula. ##STR24##

Comparative Example 4

A magnetic tape was produced in the same manner as in Example 1 exceptthat silicone compound A in the back-coating composition of Example 1was replaced by a polysiloxane modified with a fatty acid and having thefollowing average formula. ##STR25##

Comparative Example 5

A magnetic tape was produced in the same manner as in Example 1 exceptthat silicone compound A in the back-coating composition of Example 1was replaced by silicone compound F prepared in Preparation Example 6.

Comparative Example 6

A magnetic tape was produced in the same manner as in Example 1 exceptthat silicone compound A in the back-coating composition of Example 1was replaced by silicone compound G prepared in Preparation Example 7.

The magnetic tapes prepared in Examples 1 to 7 and Comparative Examples1 to 6 were evaluated with respect to the roughness for a center line ofthe surface (Ra) of the backing layer, conditions of the surface of thecoat, adhesion property of the coating onto the magnetic layer, transferproperty of the silicone compounds into the magnetic layer, durabilityunder traveling, and dropout.

The roughness for a center line of the surface (Ra) of the backing layerwas measured using a roughness measuring apparatus equipped with astylus, with a cut-off value of a 0.08 mm, according to JIS-B-0601-1982.

The condition of the surface of the coating was visually checked interms of pin holes; repelling in the surface; and uniformity of thesurface, according to the following criteria:

A: Neither pin holes nor repelling was observed

B: Some pin holes and repelling were observed.

C: Many pin holes and much repelling were observed.

After winding the magnetic tape at a constant tension on a reel andallowing it to stand in high temperature/high humidity conditions (60°C., RH 80%) for 24 hours, adhesion of the coating to the magnetic layerwas measured as an angle of the tape formed when it is rewound,according to JIS C6240. In addition, damage to the surface of thebacking layer which had been separated was observed under a microscopeand the following ratings were given.

A: No damage was found in the backing layer.

B: Separation was observed in a part of the backing layer.

C: 50% or more of the backing layer was separated.

With respect to the transfer property of the silicone compound to themagnetic layer, changes in critical surface tensions of the surfaces ofthe backing layer and the magnetic layer were investigated by measuringa contact angle before and after the adhesion test. Silicone compoundshave a low critical surface tension. Therefore, if they transfer to themagnetic layer after the adhesion test, critical surface tension of thebacking layer increases, whereas that of the magnetic layer decreases.

In the test for durability under traveling, damage to the surface of thebacking layer of the tape was checked after 200 tape passes in a tapetraveling tester. Dropout was measured using a dropout counter.

The results of the above tests are shown in Table 1. As is apparent fromTable 1, the backing layer made from a coating composition containingthe silicone compound used in the present invention does not adhere tothe magnetic layer under severe conditions of high temperature and highhumidity. Moreover, the silicone compound does not transfer to themagnetic layer after the adhesion test, and does not affect the magneticlayer. The silicone compound has excellent compatibility with othercomponents in the coating composition. Thus, film-defects such as pinholes, repelling, or crawling are not observed in the surface of thebacking layer. In addition, due to the lubricating effect of thesilicone compound, durability in traveling is improved.

                                      TABLE 1    __________________________________________________________________________                               Changes in critical              Surface                    Nonadhesiveness                               surface tension    Examples/          Backing              conditions of                         Damages of                               (dyn/cm) Durability    Comparative          layer              the backing                    Angle of                         the backing                               Backing                                   Magnetic                                        under                                             Drop-    Examples          Ra (nm)              layer separation                         layer layer                                   layer                                        traveling                                             out    __________________________________________________________________________    Example:    1     8.5 A     0°                         A     +0.3                                   -0.4 B-A  A    2     8.7 A     0°                         A     +0.2                                   -0.1 A    A    3     8.8 A     0°                         A     +0.1                                   -0.2 A    A    4     8.2 A     0°                         A     0   -0.1 A    A    5     8.0 A     0°                         A     0   0    A    A    6     9.2 A     0°                         A     +0.3                                   -0.1 B-A  A    7     18.0              A     0°                         A     +0.3                                   -0.2 B    B    Comparative Example:    1     8.4 A     >90°                         C     --  --   B    B    2     10.2              C     10°                         B     +1.3                                   -1.0 C    B    3     9.2 B-C   0°                         B     +0.8                                   -0.6 B    B    4     10.0              B     20°                         B     +4.5                                   -4.1 B    C    5     8.8 A     0°                         A     +3.5                                   -2.8 B    C    6     9.5 B-C   0°                         A     +0.3                                   -0.2 B    B    __________________________________________________________________________     Note 1: The results where the separation angle is 0° indicate the     state where no adhesion occurred.     Note 2: The larger values the changes in critical surface tension have,     the more the silicone compounds transfer to the magnetic layer.

What is claimed is:
 1. A paint for forming a backing layer which isprovided on/over one surface of a magnetic recording medium whereinon/over the other surface thereof a magnetic layer is provided,comprising:a filler containing carbon black; an organic colorant offormula (Ca) or (Cb); ##STR26## wherein Q is an organic colorantresidue; X is a direct bond, CONH--Y¹ --, --SO₂ NH--Y¹, or CH₂ NHCOCH₂NH--Y¹ -- wherein Y¹ is a substituted or unsubstituted alkylene orarylene group: Y is ##STR27## wherein R¹ and R² each independentlyrepresent substituted or unsubstituted alkyl or jointly form asubstituted or unsubstituted nitrogen heterocycle; Y² is a hydrogenatom, a halogen atom, NO₂, NH₂ or SO₃ H; k is an integer of 1-4; m is aninteger of 1-6: and n is an integer of 1-4: ##STR28## wherein Q is anorganic colorant residue; X is a direct bond, CONH--Y⁴ --, SO₂ NH--Y⁴,or CH₂ NHCOCH₂ NH--Y⁴ -- wherein Y⁴ is an alkylene or arylene group Y³is --NH or --O--; Z is hydroxy, alkoxy, or ##STR29## wherein Y⁵ is NH orO, or --NH--X--O-- wherein n is 1; R¹ and R² each independentlyrepresent substituted or unsubstituted alkyl, or jointly form asubstituted or unsubstituted nitrogen heterocycle; m is an integer of1-6; and n is an integer of 1-4; a binder; and a silicone compoundrepresented by the following formula (I): ##STR30## wherein groups A, B,and D each independently represent --CH₃ or a group -X"-Y"-Z" in whichX" represents a C₁ -C₁₁ alkylene or --(CH₂)₃ NHCO--, Y" represents analiphatic group, alicyclic group, or an aromatic group having at least 3carboxylate ester bonds therein and a number-average molecular weight Mnof from about 300 to about 3,000, and Z" represents --OH, m is aninteger from 4 to about 150, and n is an integer from 1 to 6, thecompound of formula (I) containing at least one -X"-Y"-Z" group, withthe ratio of the number of groups -X"-Y"-Z" to m being from 1:3 to1:about 30, said silicone compound being in the amount of from 0.05% to10.0% based on the non-volatile weight of the backing layer.
 2. Thepaint as defined in claim 1, wherein the carboxylate ester bonds arelinked to each other through a hydrocarbon group having 2 to 12 carbonatoms.
 3. The paint as defined in claim 2, wherein the carboxylate esterbonds are linked to each other through a hydrocarbon group having 4 to 6carbon atoms.
 4. The paint as defined in claim 1, wherein Y" ispolycaprolacton.
 5. The paint as defined in claim 1, wherein each of Aand B is X"-Y"-Z", and D is CH₃.
 6. The paint as defined in claim 1,wherein each of A and B is CH₃, and D is X"-Y"-Z".
 7. The paint asdefined in claim 1, wherein the number of the X"-Y"-Z" groups to m isfrom 1:4 to 1:16.
 8. The paint as defined in claim 1, wherein the fillerfurther comprises at least one selected from the group consisting ofgraphite, calcium carbonate, barium sulfate, titanium oxide, alpha-ironoxide, plate-like alpha-iron oxide, alumina, zirconia, silicon carbide,silicon oxide, chromium oxide, magnesium carbonate, zinc oxide, andsilica.
 9. The paint as defined in claim 1, wherein the binder is athermoplastic resin having a glass transition temperature of from -100°to 150° C. and a number-average molecular weight of from 1,000 to200,000.
 10. The paint as defined in claim 1, wherein the binder isthermosetting resin selected from phenol resins, epoxy resins, phenoxyresins, polyurethane setting resins, urea resins, melanine resins, alkydresins, acrylic resins, formaldehyde resins, silicone resins, mixturesof a polyester resin and an isocyanate prepolymer, mixtures of polyesterpolyol and polyisocyanate, and mixtures of polyurethane andpolyisocyanate.
 11. The paint as defined in claim 1, wherein the binderis a fiber resin selected from nitrocellulose, ethylcellulose, celluloseacetate butyrate, and cellulose acetate propionate.
 12. The paint asdefined in claim 1, wherein the binder is a combination of apolyurethane resin and a member selected from the group consisting ofvinyl chloride resins, vinyl chloride/vinyl acetate resins, vinylchloride/vinyl acetate/vinyl alcohol resins, vinyl chloride/vinylacetate/maleic anhydride resins, vinyl chloride/acrylic ester resins,nitrocellulose, and phenoxy resins; or a mixture of any one of thecombination with polyisocyanate.
 13. The paint as defined in claim 1,wherein the binder is selected from a combination of nitrocellulose, aurethane resin and polyisocyanate; and a combination of a vinyl chlorideresin, a urethane resin and polyisocyanate.
 14. The paint as defined inclaim 13, wherein the urethane resin is selected from polyesterpolyurethane, polyether polyurethane, polycarbonate polyurethane, andpolycaprolactone polyurethane, which may optionally have at least onepolar group selected from the group consisting of --COOM, --SO₃ M,--OSO₃ M, --PO(OM)₂, (M is hydrogen or alkali metal), --OH, NR₂, N⁺ R₃(R is hydrocarbon), epoxy, and sulfobetaine.
 15. The paint as defined inclaim 13, wherein the polyisocyanate in either combination is selectedfrom the group consisting of tolylene diisocyanate, 4,4'-diphenylmethanediisocyanate, hexamethylene diisocyanate, xylyene diisocyanate,orthotoluidine isocyanate, isophorone diisocyanate, and triphenylmethanetriisocyanate; reaction products of any one of the isocyanates and apolyalcohol; and polyisocyanates obtained from condensation reaction ofisocyanates.
 16. The paint as defined in claim 1, wherein the hydroxylvalue of the silicone compound is not less than 5 (KOH mg/g).
 17. Thepaint as defined in claim 1, wherein said organic colorant is in theamount of from 0.1% to 50% based on the weight of said filler.
 18. Thepaint as defined in claim 1, wherein said organic colorant is selectedfrom the group consisting of phthalocyanine, azo, anthraquinone,quinaquidone, dioxazine, anthrapyrimidine, anthanthron, indanthron,flavanthron, perylene, perynone, thioindigo, isoindolinone,triphenylmethane, and benzimidazolone types.
 19. The paint as defined inclaim 1, wherein the ratio of the filler to the binder is from 100:5 to100:300.
 20. A method of forming a backing layer of a magnetic recordingmedium sandwiched by said backing layer and a magnetic layer, comprisingthe steps of:(a) applying a paint to form a backing layer, said paintcomprising:a filler containing carbon black; an organic colorant offormula (Ca) or (Cb); ##STR31## wherein Q is an organic colorantresidue; X is a direct bond, CONH--Y¹ --, --SO₂ NH--Y¹, or CH₂ NHCOCH₂NH--Y¹ -- wherein Y¹ is a substituted or unsubstituted alkylene orarylene group; Y is ##STR32## wherein R¹ and R² each independentlyrepresent substituted or unsubstituted alkyl, or jointly form asubstituted or unsubstituted nitrogen heterocycle; Y² is a hydrogenatom, a halogen atom, NO₂, NH₂ or SO₃ H; k is an integer of 1-4; m is aninteger of 1-6; and n is an integer of 1-4; ##STR33## wherein Q is anorganic colorant residue; X is a direct bond, CONH--Y⁴ --, SO₂ NH--Y⁴,or CH₂ NHCOCH₂ NH--Y⁴ -- wherein Y⁴ is an alkylene or arylene group Y³is --NH or --O--; Z is hydroxy, alkoxy, or ##STR34## wherein Y⁵ is NH orO, or --NH--X--O-- wherein n is 1; R¹ and R² each independentlyrepresent substituted or unsubstituted alkyl, or jointly form asubstituted or unsubstituted nitrogen heterocycle; m is an integer of1-6; and n is an integer of 1-4; a binder; and a silicone compoundrepresented by the following formula (I): ##STR35## wherein groups A, B,and D each independently represent --CH₃ or a group -X"-Y"-Z", in whichX" represents a C₁ -C₁₁ alkylene or --(CH₂)₃ NHCO--, Y" represents analiphatic group, alicyclic group, or an aromatic group having at least 3carboxylate ester bonds therein and a number-average molecular weight Mnof from about 300 to about 3,000, and Z" represents --OH, m is aninteger from 4 to about 150, and n is an integer from 1 to 6, thecompound of formula (I) containing at least one -X"-Y"-Z"group, with theratio of the number of groups -X"-Y"-Z" to m being from 1:3 to 1: about30, said silicone compound being in the amount of from 0.05% to 10.0%based on the non-volatile weight of the backing layer; and (b) dryingthe applied paint.